In the field of graphic arts, in order to obtain satisfactory reproducibility of a dot image having a continuous graduation or a line image, an image formation system having super-high contrast photographic characteristics (particularly as having a gamma of not less than 10) is required.
For this purpose, a special developer referred to as lith developer has been conventionally employed. A lith developer contains hydroquinone as a developing agent, and, as a preservative, sulfite in the form of an adduct with formaldehyde, so as to have the free sulfite ion concentration controlled at an extremely low level, usually not more than 0.1 mol/l, in order not to inhibit infectious development. Therefore, the lith developer is extremely susceptible to air oxidation and does not withstand preservation for more than 3 days.
Use of hydrazine derivatives has been proposed for obtaining a high contrast with a stable developing solution, as disclosed in U.S. Pat. Nos. 4,224,401, 4,168,977, 4,166,742, 4,311,781, 4,272,606, 4,211,857, 4,243,739, etc. According to this technique, photographic characteristics of super-high contrast and high sensitivity can be achieved, and the stability of the developer to air-oxidation is markedly improved over earlier lith developers because addition of a sulfite to the developer at a high concentration is permissible.
However, the above-described image formation system with high sulfite condition brings about not only high sensitivity and high contrast, but also an unfavorable phenomenon of black spots, sometimes called black pepper, on non-developed areas between dots due to infection, which has presented a serious problem in photomechanical process. Formation of black spots frequently occurs when a light-sensitive material is preserved, particularly under a high temperature and high humidity condition, or when a developer undergoes fatigue with time, which generally involves decrease in sulfite ions used as preservative or increase in pH, resulting in considerable reduction of commercial value as a light-sensitive material for photomechanical process. Although many efforts have been made to overcome this problem, conventionally proposed techniques for eliminating black spots are often accompanied by reduction of sensitivity and gamma. Therefore, it has been keenly desired to develop a system free from black spots while retaining high sensitivity and high contrast.
It has proved effective for this purpose to lower the pH value of a film to 5.8 or less by adding an acidic substance as disclosed in Japanese Patent Applicaton (OPI) No. 228437/86 (the term "OPI" as used herein means a "published unexamined" Japanese Patent Application). Nevertheless, this technique is still unsatisfactory and is also found to cause another problem, particularly that the hydrazine derivative is precipitated in a coating composition before coating so that uniform activity of the hydrazine derivative cannot be obtained.